The present invention relates to xe2x80x9chybridxe2x80x9d or compound vacuum pumps which have two or more sections of different operational mode for improving the operating range of pressures and throughput; and more particularly, to oil free (dry) compound vacuum pumps.
A screw pump comprising two externally threaded or vaned rotors mounted in a pump body and adapted for counter rotation in said body with intermeshing of the rotor threads is well known. Close tolerances between the rotor threads at the points of intermeshing and with the internal surfaces of the pump body causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface of the pump body and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive because they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the inlet down to atmospheric pressure at the outlet. However, such screw pumps suffer from low pumping speeds at relatively low pressures in the order of 500 mbar or less and, to overcome this problem, they are often fitted in tandem with a separate Roots-type pump to boost the pumping speed. The pumping capacity of the Roots-type pump can be up to 10 times that of the screw pump.
An example of a screw pump in tandem with a Roots-type pump is described in EP Publication No. 0965758 where a Roots stage occupies a first chamber of a xe2x80x9chybridxe2x80x9d pump adjacent an inlet to the pump and a screw pump stage occupies a second chamber of said pump adjacent an outlet from the pump.
However, it has been found that when pumping down from high inlet pressures a significant interstage pressure can develop between the Roots and the screw pump stages. This imposes a high force on the screw pump rotors and as a consequence the pump bearings.
It is an aim of the present invention to modify the Roots-type rotors to obviate this disadvantage.
According to the present invention, a compound vacuum pump comprises a screw pump section having a first shaft and space therefrom and parallel thereto a second shaft mounted in a pump body, a first rotor mounted on the first shaft and a second rotor mounted on the second shaft, each rotor being substantially cylindrical and having formed on an outer surface at least one helical vane or thread, the helical vanes or threads intermeshing together in a first chamber in the pump body so that rotary movement of the shafts will cause a fluid to be pumped from an inlet towards an outlet, the pump additionally including a Roots-type pump section including two mating Roots-type rotors also mounted on the said shafts and adapted for counter rotation in a second chamber in the pump body located at the inlet end of the pump, and wherein each Roots-type rotor has a disc for rotation in a bore of a partition separating said first and second chambers.
Preferably, the discs are circular in cross-section and each has a diameter slightly less than the centre distance between said first and second shafts.